I beg the reader to allow me to begin this chapter with a word of warning. The reader is aware that Criticism—by which I mean Thought—may be any one of three kinds: it may be purely destructive; it may be purely constructive; or it may be both destructive and constructive at the same time. Purely destructive criticism is sometimes highly useful. If an old idea or a system of old ideas be false and therefore harmful, it is a genuine service to attack it and destroy it even if nothing be offered to take its place, just as it is good to destroy a rattlesnake lurking by a human pathway, even if one does not offer a substitute for the snake. But, however useful destructive criticism may be, it is not an easy service to render; for old ideas, however false and harmful, are protected alike by habit and by the inborn conservatism of many minds. Now, habit indeed is exceedingly useful—even indispensable to the effective conduct of life—for it enables us to do many useful things automatically and therefore easily, without conscious thinking, and thus to save our mental energy for other work; but for the same reason, habit is often very harmful; it makes us protect false ideas automatically, and so when the destructive critic endeavors to destroy such ideas by reasoning with us, he finds that he is trying to reason with automats—with machines. Such is the chief difficulty encountered by destructive criticism. On the other hand, purely constructive criticism—purely constructive thought—consists in introducing new ideas of a kind that do not clash, or do not seem to clash, with old ones. Is such criticism or thought easy? Far from it. It has difficulties of its own. These are of two varieties: the difficulty of showing people who are content with their present stock of old ideas that the new ones are interesting or important; and the great difficulty of making _new_ ideas clear and intelligible, for the art of being clear and perfectly intelligible is very, very hard to acquire and to practise. The third kind of criticism—the third kind of thought—the kind that is at once both destructive and constructive—has a double aim—that of destroying old ideas that are false and that of replacing them with new ideas that are true; and so the third kind of criticism or thought is the most difficult of all, for it has to overcome both the difficulty of destructive criticism and that of constructive thought.